Imaging beta cell mass

ABSTRACT

The present disclosure provides compositions and methods based on peptide-detectable agent conjugates that are useful for imaging beta cell mass.

CROSS REFERENCE TO RELATED APPLICATION

This application is continuation of U.S. patent application Ser. No. 14/003,959, filed Sep. 9, 2013, which is a national stage application under 35 U.S.C. § 371 of PCT/US2011/041986, filed Jun. 27, 2011, and claims priority to U.S. Provisional Patent Application No. 61/450,754, filed on Mar. 9, 2011, each of which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present invention was made with government support Under Grant Nos. P01 AI54904, RO1 67536, U24 CA092782, and 1RL9EB008539-01 awarded by the National Institutes of Health. The United States Government has certain rights in the invention.

TECHNICAL FIELD

The present disclosure describes the design and synthesis of peptide-detectable agent conjugates for imaging beta cell mass.

BACKGROUND

Beta cell mass (BCM) and the functional state of islets are critical measures in assessing the magnitude of autoimmune destruction in type 1 diabetes (T1D). Progressive loss of BCM is also responsible for the secondary failure of clinical drugs in type 2 diabetes. Serum tests such as insulin/C-peptide do not reliably measure BCM, and currently the only accepted gold standard of measurement is autopsy. Targeting receptors located on beta cells, such as the glucagon like peptide-1 receptor (GLP-1R), could prove useful as a diagnostic, preventative, and clinical tool in assessing beta cell mass. Thus, there exists a need for imaging probes that can be used to: a) better understand the history of the islet and the pathophysiology of diabetes, b) enable earlier diagnosis of T1D, c) allow monitoring of therapeutic efficacy and durability of drugs, d) reveal image-able biomarkers useful in the discovery of new therapies, e) monitor islet transplantation, and e) be used to localize beta cell derived malignancies. The compositions and methods described herein address these and other needs.

SUMMARY

The present disclosure provides, inter alia, compositions and methods of making and using peptide-detectable agent conjugates that can be used to image beta cell mass in vivo and ex vivo. The imaging of beta cell mass can be useful for measuring disease onset, progression and treatment. The peptide-detectable agent conjugates include a detectable agent, a peptide, and a linker that connects the detectable agent to the peptide. The peptide-detectable agent conjugates strongly bind to GLP-1R receptors with high specificity and thus provides a way of directly measuring beta cell mass. The detectable agent can be detected by various imaging modalities including, e.g., near infrared (NIR), single photon emission computed tomography (SPECT) scan, positron emission tomography (PET) scan, or magnetic resonance imaging (MRI) scan or other imaging modality.

Accordingly, in a first aspect the present disclosure provides a composition comprising the peptide sequence SEQ ID NO: 1, wherein the lysine at position 12 is modified with a detectable agent either directly or via a linker.

In some embodiments, the lysine at position 12 is modified with a fluorophore. In some embodiments, the fluorophore is VT-680 or VT-750.

In another aspect, the present disclosure provides a composition comprising the peptide sequence SEQ ID NO:2, wherein the amino acid at position 12 is an unnatural amino acid.

In some embodiments, the unnatural amino acid is selected from the group consisting of: 3-azido-L-alanine, 3-azido-D-alanine, 4-azido-L-homoalanine, 4-azido-D-homoalanine, (S)-2-amino-5-azidopentanoic acid, (R)-2-amino-5-azidopentanoic acid, (S)-2-amino-6-azidohexanoic acid, (R)-2-amino-6-azidohexanoic acid, 4-azido-L-phenylalanine, 4-azido-D-phenylalanine, L-propargylglycine, D-propargylglycine, (S)-2-amino-4-pentynoic acid, (R)-2-amino-4-pentynoic acid, (D)-homopropargylglycine, and (L)-homopropargylglycine.

In some embodiments, the unnatural amino acid is (S)-2-amino-4-pentynoic acid. In some embodiments, the unnatural amino acid is attached to a detectable agent.

In some embodiments, the detectable agent is attached to the unnatural amino acid through the use of a bioorthogonal conjugation reaction.

In some embodiments, the bioorthogonal conjugation reaction is a copper catalyzed “click” cycloaddition reaction or an inverse electron demand Diels-Alder cycloaddition reaction.

In some embodiments, the bioorthogonal conjugation reaction is the copper catalyzed “click” cycloaddition reaction.

In some embodiments, the conjugation reaction between peptide and linker or linker and detectable agent can be a nucleophilic substitution, a maleimide/cysteine reaction, ligation of an azide to a terminal alkyne, strained alkynes or triaryl phosphines, ligation of an aldehyde or ketone to a hydroxylamine or hydrazide. For additional examples of chemical modifications, conjugation strategies and bioorthogonal reactions see, e.g., Beatty, Acc. Chem. Res. DOI: 10.1039/C1MB05040K; Sletten et al., Angew. Chem. Int. Ed. 48:6974-6998, 2009.

In another aspect, the present disclosure provides a composition comprising the peptide sequence SEQ ID NO:3, wherein the amino acid at position 12 is cysteine.

In some embodiments, the cysteine is modified with a linker comprising a maleimide at a first end and a tetrazine at a second end.

In some embodiments, the cysteine reacts with the maleimide via a 1,2-addition reaction.

In some embodiments, the tetrazine reacts with a strained alkene attached to a detectable agent via an inverse electron demand Diels-Alder reaction.

In some embodiments, the strained alkene is trans-cyclooctene. In some embodiments, the strained alkene is norbornene.

In some embodiments, the strained alkene is trans-cyclooctene.

In some embodiments, the detectable agent comprises a fluorophore, a luminescent agent, a radioactive isotope, or a paramagnetic element.

In some embodiments, the detectable agent is a fluorophore. In some embodiments, the fluorophore is VT-680 or VT-750.

In some embodiments, the detectable agent comprises a radioactive isotope selected from the group consisting of: fluorine-18, carbon 11, nitrogen-13, zirconium-89, copper-64 gallium-67, technetium-99m, indium-111, iodine-123, xenon-133, and thallium-201 or other isotopes suitable for PET and/or SPECT imaging. In some embodiments, the detectable agent is fluorine-18.

In another aspect, the present disclosure provides methods of imaging beta cells in a subject, the method comprising: administering to the subject a peptide-detectable agent conjugate comprising the composition of any one of claims 1 to 12; and detecting the peptide-detectable agent conjugate in the subject; thereby obtaining an image of the peptide-detectable agent conjugate in the subject.

In some embodiments, the methods described herein further comprise quantifying the peptide-detectable agent conjugate in the subject, thereby quantifying beta cell mass.

In some embodiments, the methods described herein further comprise administering the peptide-detectable agent conjugate to the subject a second time, obtaining a second image, and comparing the two images to detect changes in beta cell mass.

In some embodiments, the methods described herein further comprise administering a treatment to the subject before the second time, wherein a change in the two images indicates an effect of the treatment on beta cell mass.

In some embodiments, the subject has a beta cell tumor, and the treatment is chemotherapy or surgery. In some embodiments, the tumor is an insulinoma. In some embodiments, the subject has diabetes or is pre-diabetic, and the treatment is intended to increase beta cell mass. In some embodiments, the treatment is a beta cell regeneration treatment. In some embodiments, the treatment is part of a clinical trial.

In some embodiments, the image is obtained by an imaging modality selected from the group consisting of: X-ray imaging, computed tomography (CT) imaging, single photon emission computed tomography (SPECT) imaging, positron emission tomography (PET) imaging, infrared imaging, magnetic resonance imaging (MRI), fluorescence imaging, laparoscopy, and endomicroscopy.

In some embodiments, the detectable agent is paramagnetic or radioactive. In some embodiments, the detectable agent comprises a radioactive isotope selected from the group consisting of: fluorine-18, carbon 11, nitrogen-13, zirconium-89, copper-64 gallium-67, technetium-99m, indium-111, iodine-123, xenon-133, and thallium-201 or other isotopes suitable for PET and/or SPECT imaging. In some embodiments, the detectable agent is fluorine-18.

By virtue of their design, the peptide-detectable agent conjugates described herein possess certain advantages and benefits. First, the peptide-detectable agent conjugates can be used to better understand beta cell related illnesses such as type 1 diabetes. Second, the conjugates provide a very sensitive method of detecting pre-diabetes while the disease is still in the occult phase. Because these conjugates specifically bind the GLP-1R in beta cells, these conjugates can detect beta cell loss, which is an early indicator that a subject is either pre-diabetic and/or at risk of developing diabetes. Third, these conjugates can be used to monitor the efficacy, toxicity, and durability of beta cell loss treatments during clinical trials. Also, because these conjugates can be used in clinical trials, they can facilitate the discovery of new therapies.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Methods and materials are described herein for use in the present invention; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

Other features and advantages of the invention will be apparent from the following detailed description and figures, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a table depicting the amino acid sequences and molecular weight of synthesized fluorescent Exendin-4 conjugates.

FIG. 1B is a schematic of Exending-4_(K12)-VT-680 based on the nuclear magnetic resonance structure (NMR) (left); and a molecular model of Exendin-4_(K12)-VT680 complexed with the extracellular domain of GLP-1R based on the crystal structure.

FIG. 2A is an image of an electrospray ionization mass spectrometry (ESI-MS) trace of Exendin-4 conjugated to the C-terminal of VT680 fluorophore.

FIG. 2B is an image of ESI-MS trace of Exendin-4_(K12) conjugated to VT680 fluorophore.

FIG. 2C is an image of a matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) trace of Exendin-4 and Exendin-4_(K12)-VT680.

FIG. 2D is an image of a MALDI-MS trace of tryptic digest fragments of Exendin-4 and Exendin-4_(K12)-VT680.

FIG. 3A is a table of the amino acid sequences and molecular weight of synthesized Exendin-4 in which the lysine at position 12 has been replaced with (S)-2-amino-4-pentynoic acid, and Exendin-4 in which the lysine at position 12 has been replaced with (S)-2-amino-4-pentynoic acid conjugated to a VT750 fluorophore.

FIG. 3B is a schematic model of the bioorthogonal copper-catalyzed click reaction between Exendin-4 where the lysine at position 12 has been replaced with (S)-2-amino-4-pentynoic acid and azide-functionalized VT750 fluorophore.

FIG. 3C is an image of a MALDI-MS trace of Exendin-4_(X12) (where the lysine at position 12 has been replaced with (S)-2-amino-4-pentynoic acid) and Exendin-4_(X12) conjugated to a VT-750 fluorophore.

FIG. 3D is an image of the negative (top trace) and positive (bottom trace) polarized high performance liquid chromatography-electrospray ionization mass spectroscopy (HPLC-ESI MS) of Exendin-4_(X12) conjugated to a VT750 fluorophore.

FIG. 3E is an image of an absorption and emission spectra of Exendin-4_(X12) conjugated to a VT750 fluorophore.

FIG. 4A is a line graph of dose-response curves of Exendin-4_(K12) conjugated to a VT680 fluorophore where GLP-1R is either overexpressed (HEK/hGLP-1R) or naturally expressed (MIN6), and of a GLP-1R negative (NIH 3T3) cell line.

FIG. 4B is a bar graph of the blocking of Exendin-4_(K12) conjugated to VT680 (10 nM) with Exendin-4 (100 nM).

FIG. 4C is an image of live cell microscopy. The cell lines HEK/hGLP-1R, MIN6 or NIH 3T3 were treated with Exendin-4_(K12) conjugated to VT680 (10 nM) and then imaged (Scale bar: 50 μm).

FIG. 4D is an image of a Western Blot of HEK/hGLP-1R, MIN6 and NIH 3T3 cell lines against GLP-1R and tubulin.

FIG. 5A is a line graph of cell binding assays of different concentrations of Exendin-4_(X12) conjugated to VT750 against HEK/hGLP-1R (GLP-1R overexpressing), MIN6 (naturally expressing GLP-1R) and NIH 3T3 (GLP-1R negative).

FIG. 5B is a line graph of in vitro blocking experiments of Exendin-4_(X12) conjugated to VT750/GLP-1R binding (E4_(X12)-VT750=10 nM) with different concentrations of Exendin-4.

FIG. 5C is fluorescence image of cells following incubation with Exendin-4_(X12) conjugated to VT750 alone (left column), or after pre-incubation with excess Exendin-4 (right column) (Scale bar: 20 μm).

FIG. 6 is an in vivo image of Exendin-4_(K12) conjugated to VT680 in a pancreatic islet of a live NOD MIP-GFP mouse; GFP-expressing pancreatic beta cells are depicted in the top left image; Exendin-4_(K12) conjugated to VT680 accumulation (dose: 0.4 nmol/g) is depicted in the top right image; fluorescent vascular agent is depicted in the bottom left image; and merging of these three images is depicted in the bottom right. All images acquired with 20× objective in anesthetized live mice (Scale bar: 50 μm).

FIG. 7 is a fluorescence image of in vivo time course imaging of probe accumulation in an islet. GPF channel (488 nm) fluorescence images are depicted in the top row; Exendin-4_(K12) conjugated to VT680 channel (680 nm) fluorescence images are depicted in the middle row, and vasculature (750 nm) fluorescence images are depicted in the bottom row (all images with 10× objectives; scale bar: 200 μm).

FIG. 8 is a fluorescence image of in vivo time course of the conjugate accumulation in an islet. GPF channel (488 nm) fluorescence images are depicted in the top row; Exendin-4_(K12) conjugated to VT680 channel (680 nm) fluorescence images are depicted in the middle row, and vasculature (750 nm) fluorescence images are depicted in the bottom row (all images with 10× objectives; scale bar: 200 μm).

FIGS. 9A-9D are intravital confocal images of a pancreatic islet (arrows) in a live MIP-GFP mouse. FIG. 9A is an image of the GFP signal from MIP-GFP-positive pancreatic beta cells; FIG. 9B is an image of Exendin-4_(X12)-VT750 (0.2 nmol/g); FIG. 9C is a fluorescence image of a vascular agent; and FIG. 9D is a composite of the images depicted in FIGS. 9A-9D. All images were acquired with a 20× objective in anesthetized live mice.

FIG. 9E is a line graph of the increase in islet signal following a dose of systemic Exendin-4_(X12) conjugated to VT750 injection.

FIG. 9F is a line graph of the blood half-life measurement of Exendin-4_(X12) conjugated to VT750 (0.2 nmol/g) injected into a MIP-GFP mouse (scale bar: 100 μm).

FIG. 10A is a fluorescence image of a single islet in an MIP-GFP mouse.

FIG. 10B is a fluorescence image of a single islet in an MIP-GFP mouse injected with Exendin-4_(X12) conjugated to VT750.

FIGS. 10C and 10D are images of a single islet with H&E-staining and insulin-staining, respectively.

FIG. 10E is a line graph of the correlation of islet sizes estimated via the MIP-GFP reporter (488 nm) versus the GLP-1R probe Exendin-4_(X12) conjugated to VT750 fluorophore (750 nm).

FIG. 10F is a line graph of the correlation of islet sizes observed with the MIP-GFP reporter (488 nm) versus signal intensity observed at 750 nm in the same area.

FIGS. 11A and 11B are surface reflectance microscopy images of typical staining patterns observed for pancreata injected with Exendin-4_(X12) conjugated to VT750 (0.1 nmol/g, 40 minutes, mouse perfused and pancreas flushed); and FIG. 11B is an image of a higher magnification of a section from FIG. 11A.

FIG. 11C is an image of quantitative fluorescence signals (750 nm) observed in pancreata of diabetic and non-diabetic mice injected with Exendin-4_(X12)-VT750 (0.1 nmol/g, 40 minutes, mouse perfused and pancreas flushed; P<0.0001). Light grey: STZ treated but non-diabetic mice (blood glucose levels <200 mg/dL). Darker grey: overt diabetic (blood glucose levels >300 mg/dL). Blood glucose levels are indicated by 0 symbols (P<0.0001).

FIG. 11D is a bar graph of beta cell mass, as estimated from histology of untreated mice and mice treated with different concentrations of STZ.

FIG. 11E is a line graph of the correlation of beta cell mass quantification data from immunohistochemical staining and from Exendin-4_(X12)-VT750 fluorescence measurements.

FIG. 12A is a drawing of an intravital imaging system.

FIG. 12B is a photographic image of an endoscopic imaging probe.

FIG. 12C is a photographic image of the imaging probe head (outer diameter: 1.6 mm, probe head indicated in FIG. 12B by box).

FIG. 12D is an image of the field of view under white light conditions.

FIG. 12E is an image of near infrared (NIR) signal in islets of the same area as in FIG. 12D.

FIG. 12F is a composite image of the images depicted in FIGS. 12D and 12E.

FIG. 13A is a table of the amino acid sequences and molecular weight of synthesized Exendin-4 in which the lysine at position 12 has been replaced with cysteine (second row), then modified with a linker having a tetrazine (E4_(Tz12)) (third row), and then conjugated to an ¹⁹F isotope (E4_(TzTCO12)-¹⁹F) (fourth row), and also conjugated to an ¹⁸F radioactive isotope (E4_(TzTCO12)-¹⁸F) (bottom row).

FIG. 13B is a synthetic scheme of the preparation of the linker having a maleimide at a first end and a tetrazine at a second end (3), attachment to the cysteine at position 12 of Exendin-4 (4).

FIG. 13C is the bioorthogonal conjugation of structure 4 with a strained alkene having a ¹⁸F via inverse electron demand Diels-Alder reaction to form the peptide-detectable agent conjugate 5 (E4_(TzTCO12)-18F).

FIGS. 14A and 14B are images of electrospray ionization liquid chromatography/mass spectrometry (ESI LC/MS) of Exendin-4_(Tz12) of the positive polarization and the negative polarization, respectively.

FIG. 15A is a bar graph of the biodistribution of Exendin-4 conjugated to ¹⁸F in B6 mice.

FIG. 15B is a bar graph of the biodistribution of Exendin-4_(X12) conjugated to VT750 in B6 mice.

FIG. 16A is a fluorescence image of the pancreas of a MIP-GFP mouse injected with 50 μCi of Exendin-4 conjugated to 18F. FIG. 16A is a GFP reflectance image; FIG. 16B is an autoradiography image; and FIG. 16C is a white field image.

FIG. 17 shows chemical modification to cysteine or lysine at position 12 of Exendin-4.

DETAILED DESCRIPTION

The present invention provides compositions and methods for imaging beta cell mass ex vivo and in vivo, which include a detectable agent, a peptide, and a linker that connects the detectable agent to the peptide. The peptide can be a derivative of Exendin-4 that has been modified at the 12 position to allow for the attachment of a detectable agent. The peptide-detectable agent conjugates can then be imaged in vivo or ex vivo using known imaging modalities. For example, the peptide-detectable agent conjugates described herein exhibit high beta cell specificity within the pancreas in vivo. Thus, these peptide-detectable agent conjugates can be used to noninvasively image the whole pancreas by detecting the accumulation of the peptide-detectable agent conjugates in beta cells. These peptide-detectable agent conjugate can be detected, e.g., by NIR, SPECT, PET, or MRI, and facilitate the noninvasive detection of beta cell mass changes. In vivo, these peptide-detectable agent conjugates can also be imaged using laparoscopic imaging and endomicroscopy.

Peptide-Detectable Agent Conjugates

The peptide-detectable agent conjugates include a detectable agent, an Exendin-4 derived peptide, and a linker that connects the detectable agent to the peptide (see FIG. 1A). The peptide can be modified at the lysine of position 12 either by replacing the lysine with a natural amino acid, or by replacing the lysine-12 with an unnatural amino acid. The natural or unnatural amino acid can then be further modified with a linker that can facilitate the attachment of a detectable agent.

Exendin-4 Derivatives

In some embodiments, the lysine at position 12 of Exendin-4 can be replaced by another amino acid. In some embodiments, the lysine at position 12 of Exendin-4 can be chemically modified to attach a linker. This linker can have a detectable agent attached to it or the linker can be further reacted to attach a detectable agent.

Replacement of Lysine-12 of Exendin-4

In some embodiments, the lysine at position 12 of Exendin-4 can be replaced with a cysteine. In some embodiments, the thiol group on the cysteine can be chemically modified to attach a detectable linker (see entries 1-3 of FIG. 17). In some embodiments, the thiol group can first be chemically modified to attach a linker, and then further modified to attach another group containing the detectable agent. For example, the thiol group of cysteine can undergo disulfide exchange to form mixed disulfides as well as alkylation with alkyl halides or Michael addition with α,β-unsaturated carbonyl compounds to yield thioethers.

In some embodiments, the lysine at position 12 of Exendin-4 can be replaced with a tyrosine. The phenol moiety of tyrosine can be modified through electrophilic aromatic substitution reactions with diazonium salts, iodine, or nitrous acid.

Other examples can include replacing lysine-12 with glutamate and aspartate residues and then modifying the amines located on glutamate and aspartate via carbodiimides. Additionally, lysine-12 can be replaced with histidine and can then be modified with pyrocarbonates.

In other embodiments, lysine-12 can be replaced with azides or alkynes and then be modified with alkynes or azides.

In other embodiments, lysine-12 can be replaced with strained alkynes or azides and then modified with azides or strained alkynes.

In other embodiments, lysine-12 can be replaced with azides or triaryl phosphines and then modified with triaryl phosphines and azides (Staudinger ligation).

In other embodiments, lysine-12 can be replaced with tetrazines or strained alkenes and then modified with strained alkenes or tetrazines.

In other embodiments, lysine-12 can be replaced with tetrazoles or alkenes and then reacted with alkenes or tetrazoles using a 1,3-dipolar cycloaddition.

In other embodiments, lysine-12 can be replaced with an alkene and then reacted with other alkenes using cross metathesis reactions.

For additional examples of chemical modifications, conjugation strategies and bioorthogonal reactions see, e.g., Beatty, Acc. Chem. Res. DOI: 10.1039/C1MB05040K; Sletten et al., Angew. Chem. Int. Ed. 48:6974-6998, 2009.

Chemical Modification of Existing Lysine-12 of Exendin-4

In some embodiments, the existing Lysine at position 12 of Exendin-4 is not replaced but instead reacted with activated esters, sulfonyl chlorides, isocyanates, or isothiocyanates to afford amides, sulfonamides, ureas, or thioureas, respectively (see entries 4-7 in FIG. 17) to provide a moiety for attachment of the detectable agent. Lysine residues can also undergo reductive amination reactions with aldehydes to form secondary or tertiary amines (see FIG. 17). For additional examples of chemical modifications to amino acids see, e.g., Sletten, et al. Angew. Chem. Int. Ed. 48: 6974-6998, 2009; Glazer, et al. Annu. Rev. Biochem. 39: 101, 1970; M. B. Francis in Chemical Biology (Eds.: S. L. Schreiber, T. Kapoor, G. Wess), Wiley-VCH, Weinheim, p. 593, 2007; S. D. Tilley, N. S. Joshi, M. B. Francis in The Wiley Encyclopedia of Chemical Biology (Ed.: T. Begley), Wiley-VCH, Weinheim, 2008; G. T. Hermanson, Bioconjugate Techniques, 1st ed., Academic Press, San Diego, Calif., 1996, all of which have been incorporated by reference in their entireties.

Unnatural Amino Acids

In some embodiments, the lysine at position 12 is replaced with an unnatural amino acid that may already possesses a chemical group that can react in a bioorthogonal conjugation reaction, e.g., the copper catalyzed “click” reaction or the inverse electron demand hetero-Diels Alder reaction. For example, the unnatural amino acid can have a reactive group that can react via a bioorthogonal conjugation reaction with a reactive group located on a linker attached to a detectable agent. In some embodiments, the unnatural amino acid has an azide and the linker has an alkyne, and the azide and the alkyne are reactive partners for the copper catalyzed “click” reaction. Alternatively, the alkyne is located on the unnatural amino acid and the azide is located on the linker attached to the detectable agent. Exemplary unnatural amino acids that can participate in the copper catalyzed “click” reaction are described in the Iris Biotech GMBH catalog that can be found at 2.imimg.com/data2/BB/AF/MY-2981732/iris-biotech-gmbh.pdf.

Linker

In some embodiments, a linker can be attached to the amino acid or unnatural amino acid. The term “linker” as used herein refers to a group of atoms, e.g., 0-50000 atoms, and can be comprised of the atoms or groups of atoms such as, but not limited to, hydrogen and carbon, e.g, methylene (—CH₂—), amino, amido, alkylamino, oxygen, sulfur, sulfoxide, sulfonyl, carbonyl, and imine. The linker can also comprise part of a saturated, unsaturated or aromatic ring, including polycyclic and heteroaromatic rings wherein the heteroaromatic ring is an aryl group containing from one to four heteroatoms, N, O or S. Specific examples include, but are not limited to, unsaturated alkanes, polyethylene glycols, and dextran polymers or dendrimers. The linker must not interfere with the imaging methods or with the bioorthogonal conjugation reactions described herein.

In some embodiments, the linker is not linear. For example, the linker can be a crosslinked polymer, biopolymer or dendrimer or nanoparticle, allowing attachment of one or more peptides to one or more detectable agents increasing the affinity of the peptide via multivalency, increasing the half-life, and allowing larger loading of detectable agents, which do not necessarily have to be identical and can be used to visualize beta cell mass or insulinomas with multiple imaging modalities.

In some embodiments, the linker has a first end with a reactive group that can attach to the amino acid or unnatural amino acid. For example, the amino acid can be cysteine and the linker can have a maleimide at the first end that reacts with the thiol on the cysteine via 1,2-addition (Michael Reaction) to form a carbon-sulfur bond (see FIG. 17, entry 3). The linker can have a second end that has a detectable agent already attached to the linker. Alternatively, the second end of the linker can further be coupled with a reactive group which is part of the detectable agent moiety. This coupling reaction can occur via a bioorthogonal conjugation reaction, e.g. a copper catalyzed “click” reaction or an inverse electron demand Diels-Alder cycloaddition, as described herein. In some embodiments, the cysteine can react with a linker that has a maleimide at a first end of the linker to form a carbon-sulfur bond between the thiol group of the cysteine residue and the linker. The linker can have a tetrazine located on the second end. This tetrazine can then react, via an inverse electron demand Diels-Alder cycloaddition, with a strained alkene attached to a detectable agent. See, e.g. FIGS. 13B and 13C.

In some embodiments, the first end of the linker can have an activated ester, e.g., an NHS-ester, which can form an amide bond with the amino group of the existing lysine residue at position 12. The detectable agent can be located on the second end of the linker. In some embodiments, the first end of the linker can have an azide, which can react with the alkyne group of the unnatural amino acid, (S)-2-amino-4-pentynoic acid at position 12 of Exendin-4, via a copper-catalyzed “click” reaction, and the detectable agent can be located at the second end of the linker. See, e.g., FIG. 3B.

Bioorthogonal Conjugation of Amino Acid to Detectable Agent

Inverse Electron Demand Diels-Alder Reaction

The compositions and methods described herein include the use of bioorthogonal conjugation reactions such as the inverse electron demand Diels-Alder reaction. This reaction employs a diene and a dienophile. The reaction of a diene (e.g., a substituted tetrazine) with a dienophile (e.g., an alkene or alkyne), produces an unstable cycloadduct that subsequently undergoes a retro-Diels-Alder cycloaddition reaction to produce dinitrogen as a byproduct and the desired dihydropyrazine (after reaction with an alkene) or pyrazine (after reaction with an alkyne) products (shown below in Scheme 1). See e.g., Sauer et al., Chem Ber 998: 1435-1445, 1965, which is incorporated by reference in its entirety. The dihydropyrazine product may undergo an additional oxidation step to generate the corresponding pyrazine.

A variety of tetrazines and dienophiles including cyclic and linear alkenes or alkynes have been studied in this reaction. Selection of the appropriate reaction partners, allows for tuning of the coupling rate by several orders of magnitude. (Balcar J et al., 1983, Tet Lett 24:1481-1484; Thalhammer F et al., 1990, Tet Lett 47:6851-6854). See also US 2006/0269942, WO 2007/144200, and US 2008/0181847. Bioconjugation methods using inverse electron demand Diels-Alder cycloadditions between tetrazines and highly strained dienophiles such as norbornene and trans-cyclooctene are known in the literature, however the tetrazine used has limited stability to aqueous media. (Blackman et al., 2008, J Am Chem Soc, 130, 13518-9; Devaraj et al., 2009, Angew Chem Int Ed Engl, 48, 7013-6; Devaraj et al. Bioconjug Chem, 19: 2297-9, 2008; Pipkorn et al., J Pept Sci, 15, 235-41, 2009).

In some embodiments, the detectable agent carries a functional group such as an amine, alcohol, carboxylic acid or ester, or other group of atoms that can undergo a chemical reaction allowing attachment to the diene or dienophile. Alternatively or in addition, the dienophile or heterodienophile (which can be, e.g., an alkene, alkyne, nitroso, carbonyl or imine) possesses a reactive functional group for attachment to the detectable agent. Thus, the reactive functional group on the detectable agent and/or diene/dienophile undergoes a chemical reaction to form a link between the detectable agent and the diene or dienophile.

In some embodiments, the diene or dienophile can be incorporated into a non-natural amino acid as the side chain. One of skill in the art could readily synthesize such compounds. For example, the side chain of phenylalanine or tyrosine could be replaced with a diene, e.g., a tetrazine; a dienophile, e.g., a trans-cyclooctene or norbornene, which can replace the side chain of phenylalanine, tyrosine, isoleucine, leucine, or tryptophan. These new unnatural amino acids can then be used similarly to known non-natural amino acids, e.g., a peptide or protein can be produced that include the diene or dienophile already incorporated into the primary structure of the peptide or protein.

Dienes

In some embodiments, the diene can be a substituted tetrazine or other heteroaromatic ring system with at least two nitrogens adjacent to each other, and which is a highly reactive participant in the inverse electron demand Diels-Alder reaction. The diene can be linked to the amino acid or unnatural amino acid through the use of a linker. In this embodiment, the diene possesses a reactive group such as an amine, alcohol, carboxylic acid, ester, or activated ester, or other group that can undergo a chemical reaction with the reactive moiety on the amino acid or unnatural amino acid to form a link between the two. For example, the thiol group of a cysteine at position 12 of Exendin-4 can undergo a 1,2-addition reaction with a linker having a maleimide at the first end to install a linker onto the cysteine group. The linker at the second end can have a tetrazine moiety ready for reaction with a dienophile located on the detectable agent (see FIG. 13B, compound 4). Dienes useful in the present disclosure include but are not limited to aromatic ring systems that contain two adjacent nitrogen atoms, for example, tetrazines, pyridazines, substituted or unsubstituted 1,2-diazines. Other 1,2-diazines can include 1,2-diazines annelated to a second π-electron-deficient aromatic ring such as pyrido[3,4-d]pyridazines, pyridazino[4,5-d]pyridazines, and 1,2,4-triazines. Pyridazines can also be fused with a five-membered heterocycle such as imidazo[4,5-d]pyridazines and 1,2,3-triazolo[4,5-d]pyridazines. In some preferred embodiments, the diene is an asymmetrical tetrazine as described herein, e.g., 3-(p-Benzylamino)-1,2,4,5-tetrazine (Structure I).

Dienophiles

Dienophiles useful in the present methods and compositions include but are not limited to carbon containing dienophiles such as alkenes or alkynes, or compounds containing nitroso, carbonyl or imine groups. In some embodiments, the dienophile is a strained dienophile. As used herein, a “strained” dienophile has a dihedral angle that deviates from the idealized 180 degree dihedral angle. Alternatively, non-strained dienophiles (e.g., sytrenes) and/or electron rich electrophiles (e.g., eneamines or vinyl ethers), can also be used with nitroso compounds. Alkenes as used herein refer to an alkyl group having one or more double carbon-carbon bonds such as an ethylene, propylene, and the like. Alkenes can also include cyclic, ring-strained alkenes such as trans-cyclooctene or norbornene carrying a double bond which induces significant ring strain and is thus highly reactive. Alkenes can also include more complex structures such as indoles and azaindoles, electron rich enamines. Heterodienophiles containing carbonyl, nitroso or imine groups can also be used. In some embodiments, the dienophile is a trans-cyclooctenol, e.g., (E)-cyclooct-4-enol.

In some embodiments, the detectable agent is chemically attached to the dienophile. For example, the strained alkene is chemically coupled to a detectable agent, e.g., a trans-cyclooctene modified with a radioactive isotope, e.g., ¹⁸F (see Structure II below).

Copper Catalyzed “Click” Reactions

The compositions and methods described herein also include the use of copper catalyzed “click” reaction pairs that include an azide and an alkene. In other embodiments, the compositions and methods described herein can also include the use of an aza-ylide and an ester, i.e., Staudinger ligation. See, e.g. Prescher et al., Nature 430(7002): 873-877, 2004; Rostovtsev et al., Angew Chem Int Ed 41(14): 2596-2599, 2002. Bioorthogonal “click” chemistries which may require catalysts or might be catalyst free are widely used in chemical biology for a myriad of applications such as activity based protein profiling, crosslinking of proteins, monitoring cell proliferation, generation of novel enzyme inhibitors, monitoring the synthesis of newly formed proteins, protein target identification, and studying glycan processing.

Other examples include the use of “click” chemistry to increase the signal of the detectable agent. A number of elegant probes have been introduced whose fluorescence increases after azide-alkyne cycloaddition, tetrazine-trans-cyclooctene, inverse electron demand cycloaddition, or for example staudinger ligation coupling reactions (Sivakumar et al., 2004, Org Lett, 6, 4603-6; Zhou and Fahrni, 2004, J Am Chem Soc, 126, 8862-3; Hangauer and Bertozzi, 2008, Angew Chem Int Ed Engl, 47, 2394-7; Lemieux et al., 2003, J Am Chem Soc, 125, 4708-9; Devaraj et al., 2010, Angew Chem Int Ed Engl). Most, but not all of these strategies either require a reactive group intimately attached to the fluorophore or take advantage of a FRET based activation requiring appendage of an additional molecule that can act as an energy transfer agent.

Detectable Agents

Examples of detectable agents include various organic small molecules, inorganic compounds, nanoparticles, enzymes or enzyme substrates, fluorescent materials, luminescent materials, bioluminescent materials, chemiluminescent materials, radioactive materials, and contrast agents. Examples of suitable enzymes include horseradish peroxidase, alkaline phosphatase, beta-galactosidase, or acetylcholinesterase; examples of suitable fluorescent materials include boron-dipyrromethene (BODIPY®), 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionic acid (BODIPY® FL), 6-((4,4-difluoro-1,3-dimethyl-5-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene-2-propionyl)amino)hexanoic acid, succinimidyl ester (BODIPY® TRM-X), Oregon Green 88, 6-(((4,4-difluoro-5-(2-pyrrolyl)-4-bora-3a,4a-diaza-s-indacene-3-yl)styryloxy)acetyl)aminohexanoic acid, succinimidyl ester (BODIPY® 650/665-X), 7-N,N-diethylaminocoumarin, VIVOTAG 680 (an amine-reactive near-infrared fluorochrome, from VisEn Medical), VIVOTAG-S 750 (VT750-NHS ester), umbelliferone, fluorescein, fluorescein isothiocyanate, rhodamine, dichlorotriazinylamine fluorescein, dansyl chloride or phycoerythrin.

Examples of luminescent materials include luminol. Examples of bioluminescent materials include luciferase, luciferin, and aequorin.

Examples of suitable radioactive material include ¹⁸F, ⁶⁷Ga, ^(81m)Kr, ⁸²Rb, ¹¹¹In, ¹²³I, ¹³³Xe, ²⁰¹Tl, ¹²⁵I, ³⁵S, ¹⁴C, or ³H, ^(99m)Tc (e.g, as pertechnetate (technetate(VII), TcO₄ ⁻) either directly or indirectly, or other radioisotope detectable by direct counting of radioemmission or by scintillation counting.

In addition, contrast agents, e.g., contrast agents for MRI or NMR, for X-ray CT, Raman imaging, optical coherence tomogrpahy, absorption imaging, ultrasound imaging, or thermal imaging can be used. Exemplary contrast agents include gold (e.g., gold nanoparticles), gadolinium (e.g., chelated Gd), iron oxides (e.g., superparamagnetic iron oxide (SPIO), monocrystalline iron oxide nanoparticles (MIONs), and ultrasmall superparamagnetic iron oxide (USPIO)), manganese chelates (e.g., Mn-DPDP), barium sulfate, iodinated contrast media (iohexol), microbubbles, or perfluorocarbons can also be used.

In some embodiments, the detectable agent is a non-detectable pre-cursor that becomes detectable upon activation. Examples include fluorogenic tetrazine-fluorophore constructs (e.g., tetrazine-BODIPY FL, tetrazine-Oregon Green 488, or tetrazine-BODIPY TMR-X) or enzyme activatable fluorogenic agents (e.g., PROSENSE (VisEn Medical))

When the compounds are enzymatically labeled with, for example, horseradish peroxidase, alkaline phosphatase, or luciferase, the enzymatic label is detected by determination of conversion of an appropriate substrate to product.

In vitro assays in which these compositions can be used include enzyme linked immunosorbent assays (ELISAs), immunoprecipitations, immunofluorescence, enzyme immunoassay (EIA), radioimmunoassay (RIA), and Western blot analysis.

Using the Peptide-Detectable Agent Conjugates

Methods of Detecting Peptide-Detectable Agent Imaging Conjugates

The peptide-detectable imaging conjugates described herein can be imaged using methods known in the art. For example, imaging can be achieved in living animals, organs, or tissues, using e.g. near infrared (NIR), MR imaging (MRI), positron emission tomography (PET), single photon computerized tomography (SPECT), or other whole body imaging modalities. The detectable agent of the peptide-detectable agent imaging conjugate can be imaged by these whole body imaging modalities to detect beta cell mass. For example, these peptide-detectable agent imaging conjugates can be detected by traditional fluorescence imaging techniques allowing for the facile tracking of the peptide-detectable agent imaging conjugates by fluorescence microscopy or flow cytometry using methods known in the art, e.g., as described in US 2005/0249668, the content of which is incorporated by reference in its entirety.

In some embodiments, the peptide-detectable agent conjugates are imaged in vivo using laparoscopy and endomiscroscopy. For example, the use of laparoscopy permits the facile, real-time imaging and localization of tumors labeled with fluorescent proteins. See, e.g. Cao et al. Surgical Endoscopy 25(1): 48-54, 2011, the content of which is incorporated by reference in its entirety. In some embodiments, laparoscopy can be used to diagnose pancreatic cancer by imaging the accumulation of the peptide-detectable agent conjugates on the beta cells. See, e.g., Friess et al. Journal of the American College of Surgeons 186(6): 675-682, 1998, the content of which is incorporated by reference in its entirety. In some embodiments, the use of laparoscopy as an imaging modality can be useful during surgery to locate insulinomas. See, e.g. Fink et al. Journal of Gastrointestinal Surgery 15(7): 1092-7, 2011, the content of which is incorporated by reference in its entirety. In some embodiments, the peptide-detectable agent conjugates can be imaged using fiber optic endomicroscopy. See, e.g. Wu et al. Optics Express 17(10): 7907-7915, 2009; and Bao et al. Optics Express 18(2): 1255-1260 (2010), the contents of which are incorporated by reference in their entireties.

Imaging the Peptide-Detectable Agent Conjugates

The compositions and methods described herein can be imaged using a variety of modalities that are known to one of skill in the art. Detection methods can include both imaging ex vivo and in vivo imaging methods, e.g., immunohistochemistry, bioluminescence imaging (BLI), Magnetic Resonance Imaging (MRI), positron emission tomography (PET), electron microscopy, X-ray computed tomography, Raman imaging, optical coherence tomography, absorption imaging, thermal imaging, fluorescence reflectance imaging, fluorescence microscopy, fluorescence molecular tomographic imaging, nuclear magnetic resonance imaging, X-ray imaging, ultrasound imaging, photoacoustic imaging, lab assays, or in any situation where tagging/staining/imaging is required.

In Vivo Imaging

The compounds and compositions described herein can be used in in vivo imaging methods to detect, quantify and evaluate beta cell mass. In general, such methods include administering to a subject one or more peptide-detectable agent conjugates described herein; optionally allowing conjugate to distribute within the subject; and imaging the subject, e.g., by fluoroscopy, radiography, computed tomography (CT), MRI, PET, SPECT, laparoscopy, endomicroscopy, or other whole body imaging modality to detect the presence of beta cell mass. Furthermore, it is understood that the methods (or portions thereof) can be repeated at intervals to evaluate the subject and detect any changes in beta cell mass over time. Information provided by such in vivo imaging, for example, the presence, absence, or level of emitted signal, can be used to detect and/or monitor the loss of beta cell mass or increase of beta cell mass, e.g., after medical treatment.

A number of preclinical and clinical applications for this specific conjugate can be envisioned. For example, the peptide-detectable agent conjugates described here can be used: 1) for the early detection of insulinomas, a deadly cancer evolving from healthy pancreatic beta cells; and 2) as a diagnostic tool for the early detection of pancreatic beta cell loss (i.e., the presence of an abnormally low beta cell mass), resulting in type 1 diabetes, a disease affecting millions of individuals in the United States of America alone. Early detection of pancreatic beta cell loss will not only improve direct patient care, making it possible to diagnose patients during the state of occult diabetes (before physical symptoms occur), but it will also help to find cures and treatment for diabetic patients, since the effect of drugs on pancreatic beta cell mass can be measured directly.

In addition, in vivo imaging can be used to assess the effect of a compound or therapy on cells expressing GLP1-R, e.g., beta cells, by using the conjugates described herein, wherein the subject is imaged prior to and after treatment with the compound or therapy, and the corresponding signal/images are compared. For example, a subject with a cancer, e.g., an insulinoma, can be imaged prior to and after treatment with chemotherapy or radiation therapy to determine the response of the beta cells or GLP1-R-expressing cancer cells, e.g., insulinomas, to treatment. Example chemotherapy treatments include 5-fluorouracil (5-FU), gemcitabine, and Capecitabine (XELODA®). Other novel small molecules are being widely and actively researched. These compounds are based on classical mechanisms of action as well as biological therapies targeting novel cellular survival pathways, and include fluoropyrimidines, nucleoside cytidine analogues, platinum analogues, topoisomerase inhibitors, antimicrotubule agents, proteasome inhibitors, vitamin D analogues, arachidonic acid pathway inhibitors, histone deacytylator inhibitors, farnesyltransferase inhibitors and epidermal growth factor receptor therapies. See, e.g., Shore et al. Alimentary Pharmacology & Therapeutics, 18(11): 1049-1069, 2003. Other examples of chemotherapy treatments include cisplatin,

The methods and compositions described herein can be used to help a physician or surgeon to identify and characterize pancreatic diseases, such as diabetes and tumors of the pancreas such as insulinoma. The methods and compositions described herein can also be used in the detection, characterization, and/or determination of the localization of a disease, especially early disease, the severity of a disease or a disease-associated condition, the staging of a disease, and/or monitoring a disease. The presence, absence, or level of an emitted signal can be indicative of a disease state.

In some embodiments, the peptide-detectable agent conjugates can be used to monitor an increase in beta cell mass after treatment. For example, the peptide-detectable agent conjugates can be used to assess the effectiveness of islet transplantation, e.g., where islets are taken from the pancreas of a deceased organ donor. The islets are purified, processed, and transferred into another person. Once implanted, the beta cells in these islets begin to make and release insulin. See, e.g., the worldwide web page diabetes.niddk.nih.gov/dm/pubs/pancreaticislet/ for a description of islet transplantation. These peptide-detectable agent conjugates could be useful in monitoring beta cell mass increase after transplantation.

In Vitro Imaging

With respect to in vitro imaging methods, the compounds and compositions described herein can be used in a variety of in vitro assays. An exemplary in vitro imaging method comprises: contacting a sample, for example, a biological sample, with one or more peptide-detectable agent conjugates; allowing the conjugate(s) to interact with a biological target in the sample; optionally, removing unbound agents; illuminating the sample with light of a wavelength absorbable by a fluorophore of the agents; and detecting a signal emitted from fluorophore thereby to determine whether the agent has been activated by or bound to the biological target.

After a peptide-detectable agent conjugate has been designed, synthesized, and optionally formulated, it can be tested in vitro by one skilled in the art to assess its biological and performance characteristics. For instance, different types of cells grown in culture can be used to assess the biological and performance characteristics of the agent. Cellular uptake, binding or cellular localization of the agent can be assessed using techniques known in the art, including, for example, fluorescent microscopy, fluorescence-activated cell sorting (FACS) analysis, immunohistochemistry, immunoprecipitation, in situ hybridization and Forster resonance energy transfer (FRET) or fluorescence resonance energy transfer.

By way of example, the peptide-detectable agent conjugate can be contacted with a sample for a period of time and then washed to remove any free agents. The sample can then be viewed using an appropriate detection device such as a fluorescent microscope equipped with appropriate filters matched to the optical properties of a fluorescent agent. Fluorescence microscopy of cells in culture or scintillation counting is also a convenient means for determining whether uptake and binding has occurred. Tissues, tissue sections and other types of samples such as cytospin samples can also be used in a similar manner to assess the biological and performance characteristics of the agents. Other detection methods including, but not limited to flow cytometry, immunoassays, hybridization assays, and microarray analysis can also be used.

In some embodiments, the peptide-detectable agent conjugates can be used in an in vitro assay for detecting agents that promote beta cell differentiation.

EXAMPLES

The invention is further described in the following examples, which do not limit the scope of the invention described in the claims.

Example 1. Preparation of Fluorescence-Labelled Exendin-4 Conjugates

Materials

All reagents were purchased from Sigma-Aldrich (St. Louis, Mo.) and used without further purification unless otherwise noted. Exendin-4 was obtained from Prospec (Rehovot, Israel) or as an injectable solution (250 μg/mL, Exenatide, Byetta; Amylin/Eli Lilly). Near-infrared fluorochrome Vivotag-680 (VT680-NHS ester) was purchased from VisEn (Bedford, Mass.) or from PerkinElmer (Waltham, Mass.).

Mass Spectrometry

Liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS) analysis and HPLC-purifications were performed on a Waters (Milford, Mass.) LC-MS system. For LC-ESI-MS analyses, a Waters XTERRA® C18 5 μm column was used. For preparative runs, an ATLANTIS® Prep T3 OBD™ 5 μM column was used. MALDI-MS spectra were collected on a VOYAGER-DE™ BIOSPECTROMETRY™ Workstation spectrometer (APPLIED BIOSYSTEMS™, Foster City, Calif.).

Three peptide-detectable agent conjugates were prepared using the following methods.

Preparation of Fluorescence-Labelled Exendin-4 Conjugates

1. Preparation of Exendin-4_(K12)-VT680

To a solution of 250 μg (0.06 μmol) of Exendin-4_(K12) in 3.6 mL of a 25 mM Tris-buffered aqueous solution (pH=7.2), 1 mg VivoTag 680 (0.8 μmol) was added and the reaction mixture stirred at room temperature in the dark for 3 hours. The mixture was subjected to HPLC-purification, yielding the title compound as a blue solid (100 μg, 0.02 μmol, 33%). ESI-LC-MS m/z (%)=1769 [M−3H⁺]³⁻ (100).

2. Preparation of Exendin-4₄₀-VT680

Preparation of 11-Azido-3,6,9-Trioxaundecan-1-Amide-VT680

11-Azido-3,6,9-trioxaundecan-1-amide-VT680

To a solution of 10 mg (46 μmol) 11-Azido-3,6,9-trioxaundecan-1-amine in 300 μL 10×PBS, 2.5 mg (2 μmol) VivoTag 680 was added and the reaction solution stirred at room temperature in the dark for 2 hours. The crude mixture was directly subjected to HPLC-purification, yielding 1.5 mg (1.1 μmol, 56%) of the title compound as a blue solid. ESI-LC-MS m/z (%)=1342 [M+H⁺]⁺.

Preparation of Exendin-4_(K12)-VT-680

To a solution of 0.8 mg (0.2 μmol) Exendin-4(40-Pra) (Exendin-4 with the unnatural amino acid (S)-2-amino-4-pentynoic acid attached to the C-terminal end of Exendin-4) in 400 μL of a 25 mM Tris-buffered aqueous solution (pH=7.2), 50 μL of a 50 mM solution of L-ascorbic acid in H₂O and 50 μL of a 50 mM aqueous CuSO₄ solution in H₂O was added. 0.4 mg (0.3 μmol) of 11-Azido-3,6,9-trioxaundecan-1-amide-VT680 in 500 uL 2×PBS solution was then added and the reaction mixture was gently stirred at room temperature in the dark overnight. The crude mixture was diluted with 2 mL of 1×PBS and purified using an AMICON® Ultra 3 kDa (Millipore, Carrigtwohill, Ireland) centrifugal filter, before it was subjected to HPLC purification, yielding 0.3 mg (0.05 μmol, 25%) of the title compound as a blue film. ESI-LC-MS m/z (%)=1405 [M−4H⁺]⁴⁻ (100); 1873 [M−3H⁺]³⁻ (14).

3. Preparation of Exendin-4_(X12)-VT750

Preparation of 11-Azido-3,6,9-trioxaundecan-1-amide-VT750

11-Azido-3,6,9-trioxaundecan-1-amide-VT750

To a solution of 10 mg (46 μmol) 11-Azido-3,6,9-trioxaundecan-1-amine in dichloromethane (DCM; 1 mL), 10 mg (8.5 μmol) VivoTag-S 750 and 20 μL triethylamine were added. The reaction solution was stirred at room temperature in the dark for 3 hours. Volatiles were removed under reduced pressure and the crude mixture was subsequently subjected to HPLC-purification, yielding 3.4 mg (3.1 μmol, 36%) of the title compound as a blue solid. ESI-LC-MS(+) m/z (%)=543.0 [M+2H⁺]²⁺ (100), 1084.5 [M+H⁺]⁺ (30); ESI-LC-MS(−) m/z (%)=541.0 [M−2H⁺]²⁻ (100), 1082.5 [M−H⁺]⁻ (20).

Preparation of Exendin-4_(X12)-VT750

To a solution of 3.4 mg (0.8 μmol) Exendin-4_(X12) (X corresponding to the unnatural amino acid (S)-2-amino-4-pentynoic acid) in 400 μL of a 25 mM Tris-buffered aqueous solution (pH=7.2), 50 μL of a 50 mM solution of L-ascorbic acid in H₂O and 50 μL of a 50 mM aqueous CuSO₄ solution in H₂O were added. Subsequently, 1.2 mg (1.2 μmol) of 11-Azido-3,6,9-trioxaundecan-1-amide-VT750 in 500 μL 2× phosphate buffered saline (PBS) solution was added, and the reaction mixture was gently stirred at room temperature in the dark overnight. The crude mixture was then diluted with 2 mL of 1×PBS and purified using an AMICON® Ultra 3 kDa (Millipore, Carrigtwohill, Ireland) centrifugal filter, before being subjected to HPLC purification, yielding 2.7 mg (0.52 μmol, 64%) of the title compound as a blue film. ESI-LC-MS(+) m/z (%)=1310.2 [M+4H⁺]⁴⁺ (50), 1747.1 [M+3H⁺]³⁺ (100); ESI-LC-MS(−) m/z (%)=1745.2 [M−3H⁺]³⁻ (100).

Example 2. Characterization of Fluorescence-Labelled Exendin-4 Conjugates

MALDI-MS Spectra

To a suspension of sinapinic acid (5 μL, 10 mg/mL, AcN/H₂O=1/1, 0.5% TFA), 5 μL of fluorescence-labelled exendin-4 conjugate sample (approx. 100 μM in H₂O) was added and the carefully mixed solution purified via ZipTips (Millipore, Billercia, Mass.) prior to measurement. Each sample was also run with adrenocorticotropic hormone fragment 18-39 (ACTH fragment 18-39) as a calibration standard.

Tryptic Digest

To a solution of E4_(K12)-Fluorescence labeled conjugate (10.5 μL, approx. 10 μg), 15 μL NaHCO₃ (50 mM) and 1.5 μL dithiothreitol (DTT; 100 mM) was added, and the mixture was incubated at 95° C. for 5 minutes. Subsequently, 3 μL of IAA (100 mM) was added and the sample was incubated at room temperature for 20 minutes, before 1 μL of activated trypsin (100 ng/μL, Pierce, Rockford, Ill.) was added. This was followed by an incubation at 37° C. for 3 hours. An additional 1 μL of activated trypsin (100 ng/μL) was then added and the sample was further incubated at 37° C. for 2 hours, before being purified using ZipTips (Millipore, Billerica, Mass.) and subjected to MALDI-MS analysis.

Characterization Results from Exendin-4_(K12)-VT680 Conjugate

Near infrared fluorescent Exendin-4 analogues were created via two different strategies (FIG. 1). In the first strategy, an alkyne-modified artificial amino acid (S)-2-amino-4-pentynoic acid was attached to the C-terminal end of exendin-4, yielding the peptide Exendin-4(40-Pra). To this peptide, an azide-modified near infrared (NIR) fluorochrome 11-azido-3,6,9-trioxaundecan-1-amide-VT680 was covalently attached using standard copper catalyzed click-chemistry protocols, yielding the Exendin-4-like NIR-fluorochrome, E4₄₀-VT680. 11-azido-3,6,9-trioxaundecan-1-amide-VT680 was generated in good yields (56%) by conjugation of 11-azido-3,6,9-trioxaundecan-1-amine to the commercially available NIR fluorochrome VT680-NHS ester in the presence of triethylamine. The structural identity of the exendin-like NIR-fluorochrome Exendin-4₄₀-VT680, was confirmed using liquid chromatography-mass spectrometry (LC/MS) (FIG. 2A) analysis, which confirmed the expected mass of 5623 g/mol (m/z (%)=1405 [M−4H⁺]⁴⁻ (100); 1873 [M−3H⁺]³⁻ (14)).

The second approach was based on the direct amide coupling reaction of VT680-NHS ester with the amino groups of Exendin-4, facilitated in a Tris-HCl buffered aqueous solution. Although Exendin-4 offers multiple nucleophilic amino sites, which can all theoretically react with NHS esters at one or multiple sites, only three reaction products were detected in the crude reaction mixture via LC/MS under current reaction conditions. All of the products had the same mass of 5311 g/mol (m/z (%)=1769 [M−3H⁺]³⁻ (100); FIG. 2B), which corresponded to the addition of a single fluorochrome per Exendin-4. Three products were observed in ratios of 5/5/90. The major product (90%) was isolated using preparative HPLC, yielding the Exendin-4-like NIR fluorochrome Exendin-4_(K12)-VT680 (FIG. 1).

The mass of Exendin-4_(K12)-VT680 was analyzed using matrix assisted laser desorption ionization mass spectrometry (MALDI-MS), which confirmed the exendin-4-like NIR fluorochrome to be composed of exendin-4 and a single fluorochrome molecule (m/z=5311, FIG. 2C). Structural analysis of the compound was determined by tryptic digestion of both Exendin-4_(K12)-VT680 and Exendin-4. Whereas Exendin-4 displayed two strongly ionizing fragments at 1278 Da (H1-K12) and 948 Da (L21-K27), tryptic digest of Exendin-4_(K12)-VT680 did not result in the formation of a 1278 Da fragment, but showed a dominant peak at 3373 Da (FIG. 2D). The fragment corresponded to amino acids H1-R20 (2251 Da) plus the mass of the fluorochrome (1123 Da). Observation of a peak at 3373 Da for Exendin-4_(K12)-VT680 but not for Exendin-4 indicated that the fluorochrome was part of that fragment and that it was conjugated to amino acid K12. If the fluorochrome were not conjugated to K12, the fragment would have been digested by trypsin at the K12-position into two smaller fragments. The predominant attachment of the fluorochrome to K12 compared to other nucleophilic residues (e.g. K27 or the N-terminus) is likely explained by either sterically or electronically favored conformations of K12.

Characterization Results from Exendin-4_(K12)-VT750 Conjugate

The amino acid lysine at the 12 position (K12) of the exendin-4 sequence was used as a modification point without significant reduction of peptide/GLP-1R target binding. Based on earlier findings, this was in contradistinction to fluorochrome modification of a lysine group at the 40 position (K₄₀), which has been shown to affect target selectivity (Reiner T, et al. Bioconjug Chem 21:1362-1368, 2010), presumably due to the close proximity of this lysine group to the extracellular domain of the GLP-1R binding site. To facilitate site-specific K₁₂ labeling, a neopeptide, Exendin-4_(X12) was designed, which allows attachment of desired markers via Copper(I)-catalyzed azide-alkyne cycloaddition reactions (CuAAC) (Rostovtsev et al. Angew Chem Int Ed Engl 41:2596-2599, 2002). Based on the Exendin-4 sequence, we replaced K₁₂ with the unnatural alkyne-amino acid (S)-2-amino-4-pentynoic acid. Simple CuAAC conditions (aqueous buffer, CuSO₄, L-ascorbic acid) allowed attachment of the azide-functionalized NIR-fluorophore (VT750) to Exendin-4_(X12) (FIGS. 3A and 3B). This produced the NIR GLP-1R probe with an overall synthetic yield of 64% from starting materials. HPLC-ESI/MS and MALDI-MS spectrometry confirmed the identity of Exendin-4_(X12)-VT750, and gave mass-to-charge ratios (m/z) corresponding to the calculated masses (FIGS. 3C and 3D). The maximum absorbance/emission peaks for Exendin-4_(X12)-VT750 were 754 nm and 774 nm, respectively (FIG. 3E).

Example 3. Ex Vivo Determination of Exendin-4_(K12)-VT680 Accumulation in GLP-1R Expressing Cells

Cell Culture

Mouse insulinoma-derived MIN6 cells were used between passages 26 and 40 and grown in high-glucose Dulbecco's Modified Eagles Medium (DMEM) containing 15% (v/v) heat-inactivated fetal bovine serum (FBS), 50 U/ml penicillin, and 10 μg/ml streptomycin. Human embryonic kidney cells (HEK 293 cells), stably expressing human GLP-1R (HEK/hGLP-1R), were grown in high glucose DMEM containing 10% (v/v) heat-inactivated FBS, 50 U/ml penicillin, 10 μg/ml streptomycin, 1 mM sodium pyruvate, and 150 μg/ml G418. GLP-1R negative NIH-3T3 fibroblasts (ATCC) were grown in high glucose DMEM containing 10% (v/v) heat-inactivated FBS, 50 U/ml penicillin, and 10 μg/ml streptomycin.

Western Blot

MIN6, HEK/hGLP-1R or NIH-3T3 cells, seeded into 6-well plates were washed twice with ice-cold PBS and lysed on ice with 200 μl ice-cold RIPA lysis buffer. Protein concentrations were determined using bicinchoninic acid (BCA) protein assays (Pierce, Rockford, Ill.). Cell lysates (50 μg) were subjected to SDS-PAGE, followed by immunoblotting using a specific anti-GLP-1R antibody (Abcam), and detection with chemiluminescence (Pierce, Rockford, Ill.).

Cellular Imaging

MIN6 (2×10⁵ cells/well), HEK/hGLP-1R (7.5×10⁴ cells/well) or NIH-3T3 (7.5×10⁴ cells/well) cells were seeded into 8-well chamber slides (LabTek, Rochester, N.Y.) and incubated for 48 hours before incubation with 100 nM of E4_(K12)-Fl for the indicated time points at 37° C. Cells were washed three times with PBS before subjected to imaging. MIN6, HEK/hGLP-1R and NIH 3T3 cells were imaged on an Olympus FluoView1000 confocal laser scanning microscope with an Olympus XLPan N 25× (N.A. 1.1) water immersion objective and the FluoView 1000 Vers. 2.1a program. Exendin-4_(K12)-VT680 was imaged using a 635 nm laser, dichroic mirrors DM405/488/559/635 and SDM640 and a barrier filter BA655-755.

Dose-Response Assays

MIN6 (1×10⁵ cells/well), HEK/hGLP-1R (3×10⁴ cells/well) or NIH-3T3 (3×10⁴ cells/well) cells were seeded into 96-well plates and incubated for 48 hours before adding different concentrations of Exendin4_(K12)-VT680. After incubating for 1 hour at 37° C., cells were washed three times with PBS and the fluorescence levels were measured with a SynergyMx plate reader (Biotek) with excitation at 620 nm and emission at 690 nm. Following fluorescence measurement, proteins were extracted using RIPA lysis buffers and quantified using BCA (Pierce, Rockford, Ill.).

Blocking Experiments

MIN6 (1×10⁵ cells/well), HEK/hGLP-1R (3×10⁴ cells/well) or NIH-3T3 (3×10⁴ cells/well) cells were seeded into 96-well plates and incubated for 48 hours before incubation with either Exendin-4 (100 nM) for 1 hour at 37° C. and subsequently Exendin-4_(K12)-VT680 (10 nM) for 1 hour at 37° C., or just Exendin-4_(K12)-VT680 (10 nM) for 1 hour at 37° C. Following the incubation, cells were washed three times with PBS and the fluorescence levels were measured with SynergyMx plate reader (Biotek) with excitation at 620 nm and emission at 690 nm. Following fluorescence measurement, proteins were extracted using RIPA lysis buffers (24) and quantified using BCA (Pierce, Rockford, Ill.).

Results

In order to determine accumulation of Exendin-4_(K12)-VT680 on GLP-1R expressing cells, the compounds were tested against human GLP1-R overexpressing HEK/hGLP-1R cells (HEK/hGLP-1R), GLP-1R naturally expressing MIN6, and against GLP-1R negative NIH 3T3 cells (FIGS. 4A-4D). The expression levels of GLP-1R were then analyzed by staining the respective cell lysates with an anti-GLP-1R antibody and equal cell lysate concentrations were analyzed by staining against tubulin in western blots (FIGS. 4A-4D). Western blotting confirmed GLP-1R to be absent in NIH 3T3 cells and overexpressed in transformed HEK/hGLP-1R cells. Each cell line was incubated with different concentrations of Exendin-4_(K12)-VT680 for 1 hour. The cells were later washed and their fluorescence measured. The fluorescent signal was normalized to the number of cells present in each well. GLP-1R overexpressing HEK/hGLP-1R cells displayed the highest fluorescence, followed by insulinoma-derived MIN6 cells. GLP-1R negative NIH 3T3 cells did not show any discernible fluorescence (FIG. 4A). For HEK/hGLP-1R cells, a binding EC₅₀ of 0.3±0.2 nM was determined and for MIN6 cells, the observed EC₅₀ was 0.5±0.3 nM. These findings were corroborated using confocal microscopy (FIG. 4C). Blocking experiments with non-fluorescent Exendin-4 were subsequently conducted. Pre-treated HEK/hGLP-1R cells were observed to have a 12-fold lower fluorescence and pre-treated MIN6 cells a 5-fold lower fluorescence. Finally, we also tested the terminally labeled Exendin-4₄₀-VT680, which did not show strong preferential accumulation in GLP-1R expressing cell lines.

Example 4. Ex Vivo Determination of Exendin-4_(K12)-VT750 Accumulation in GLP-1R Expressing Cells

Cell Culture

Mouse insulinoma-derived MIN6 cells were used between passages 26 and 40, and were grown in high-glucose Dulbecco's Modified Eagle's Medium (DMEM) containing 15% (v/v) heat-inactivated fetal bovine serum (FBS), 50 U/mL penicillin, and 10 μg/mL streptomycin. Human embryonic kidney cells, stably expressing human GLP-1R (HEK/hGLP-1R) were grown in high-glucose DMEM containing 10% (v/v) heat-inactivated FBS, 50 U/mL penicillin, 10 μg/mL streptomycin, 1 mM sodium pyruvate, and 150 μg/mL of the antibiotic G418. GLP-1R negative NIH-3T3 fibroblasts (ATCC) were grown in high-glucose DMEM containing 10% (v/v) heat-inactivated FBS, 50 U/mL penicillin, and 10 μg/mL streptomycin.

Cell-Labeling Assays

MIN6 (1×10⁵ cells/well), HEK/hGLP-1R (3×10⁴ cells/well) or NIH-3T3 (3×10⁴ cells/well) cells were seeded into 96-well plates and incubated for 48 hours, before incubation with various concentrations of Exendin-4_(X12)-VT750 for 90 minutes at 37° C. Following incubation, cells were washed once with PBS and the fluorescence levels were measured with a SynergyMx (Biotek, Winooski, Vt.) plate reader at an excitation of 755 nm and emission of 775 nm. After measurement of fluorescence, proteins were extracted using radioimmunoprecipitation assay (RIPA) lysis buffers and protein levels were determined using a bicinchoninic acid (BCA) assay (Thermo Fisher Scientific, Waltham, Mass.).

Blocking Experiments

MIN6 (1×10⁵ cells/well) or HEK/hGLP-1R (3×10⁴ cells/well) cells were seeded into 96-well plates and were cultured for 48 hours. The cells were then either incubated with Exendin-4_(X12)-VT750 (10 nM) for 90 minutes at 37° C. alone, or with increasing doses of exendin-4 (0 nM-500 nM) for 1 hour at 37° C. and subsequently with Exendin-4_(X12)-VT750 (10 nM) for 90 minutes at 37° C. Following incubations, cells were imaged and proteins quantified as described above.

Cell Imaging

MIN6 (2×10⁵ cells/well), HEK/hGLP-1R (7.5×10⁴ cells/well) or NIH-3T3 (7.5×10⁴ cells/well) cells were seeded onto 8-well chamber slides (LabTek, Rochester, N.Y.) and cultured for 48 hours (at 37° C.). The cells were then either incubated with 10 nM of Exendin-4_(X12)-VT750 (90 min, 37° C.) alone, or underwent an initial incubation with 3.5 μM exendin-4 (30 minutes, 37° C., and washed 1× with growth medium) before subsequent incubation with 10 nM E4_(X12)-VT750 (90 min, 37° C.). The cells were then washed once with growth medium and once with 1×PBS. MIN6, HEK/hGLP-1R and NIH 3T3 cells were imaged using an intravital laser scanning microscope (IV100, Olympus Corporation, Tokyo, Japan).

Binding, Affinity and Inhibition Results

Cell assays were performed using the MIN6 cell line, which naturally expresses GLP-1R, HEK/hGLP-1R cells which ectopically express GLP-1R, and GLP-1R-negative NIH 3T3 cells. Incubation of each line with different concentrations of Exendin-4_(X12)-VT750 revealed increasing fluorescence for MIN6 and HEK/hGLP-1R cells, whereas no increase in fluorescence was detectable for NIH 3T3 cells (FIG. 5A). The IC₅₀-values for both MIN6 and HEK/hGLP-1R cells were 3.3±1.3 nM. In order to probe the selectivity of Exendin-4_(X12)-VT750 for GLP-1R, both MIN6 and HEK/hGLP-1R cells were first incubated with different concentrations of Exendin-4 (0 nM to 500 nM) and a fixed concentration of Exendin-4_(X12)-VT750 (10 nM, FIG. 5B). Upon pre-incubation with exendin-4, a concentration dependent decrease in E4_(X12)-VT750 fluorescence signal was observed. Indeed, pre-incubation with 500 nM Exendin-4 (a 50-fold excess of Exendin-4 relative to Exendin-4_(X12)-VT750) produced a near-quantitative reduction in fluorescence signal. These results not only show that Exendin-4_(X12)-VT750 was selectively taken up by GLP-1R-expressing cell lines, but also that its uptake could be selectively inhibited upon addition of the unlabeled analogue.

Binding of GLP-1 to GLP-1R resulted in internalization of the peptide into cells and its localization in endosomal compartments. In order to visualize the cellular localization of Exendin-4_(X12)-VT750, HEK/hGLP-1R (FIG. 5C) and MIN6 cells (data not shown) were incubated with a 10 nM solution of Exendin-4_(X12)-VT750 for 90 minutes. Cells were stained with CellTracker Green to visualize the cell boundaries. The fluorescent probe Exendin-4_(X12)-VT750, like GLP-1, could be identified inside beta cells in vesicular compartments (FIG. 5C, left column). In contrast, similar to what was seen with competitive binding assays (FIG. 5B), pre-incubation with unlabeled Exendin-4 suppressed the Exendin-4_(X12)-VT750 fluorescence signal (FIG. 5C, right column).

Example 5. In Vivo Quantification of Beta Cell Mass Using Exendin-4_(X12)-VT680

Animals

NOD MIP-GFP mice (NOD.Cg-Tg(Ins-EGFP)Hara/QtngJ), that express the fluorescent protein EGFP within the islets under the mouse insulin 1 promoter were obtained from The Jackson Laboratory (Bar Harbor, Me.). All procedures and animal protocols were approved by the subcommittee on Research Animal care at Massachusetts General Hospital.

Animal Preparation

Nondiabetic male NOD MIP-GFP mice were anesthetized with 2% isoflurane and 2 L/minute O₂. The peritoneal cavity was opened with a transverse incision in the disinfected abdominal wall. The gastric-splenic ligament was dissected and the pancreas carefully exteriorized. Robust blood flow was observed in the pancreatic arteries for the duration of each experiment and pancreas perfusion was confirmed by observation of a circulating fluorescent vascular agent (Angiosense 750, VisEn, Bedford, Mass.). The exteriorized pancreas was completely submerged in temperature-controlled lactated Ringer's solution. The temperature near the spleen was carefully monitored using an Omega HH12A thermometer with fine wire thermocouples (Omega Engineering Inc., Stamford, Conn.) and kept at 37° C. A tail vein catheter, made using a 30 G×½″ needle inserted into CLAY ADAMS® Polyethylene10 tubing, was inserted prior to surgery. Subsequently, 200 μL (4 nmol/200 μL) of Exendin-4_(K12)-VT-680 and 10 minutes after the initial injection, another 200 μL (4 nmol/200 μL) of Exendin-4_(K12)-VT680 was injected via the tail vein catheter.

Intravital Laser Scanning Microscopy

Images were collected with a prototypical intravital laser scanning microscope (IV100, Olympus Corporation, Tokyo, Japan) (26) using an Olympus 4× (UplanSApo N.A. 0.16), and 20× (UplanFL N.A. 0.5) objective and the Olympus FluoView FV300 version 4.3 program. Samples were excited at 488 nm with an air-cooled argon laser (Melles Griot, Carlsbad, Calif.) for visualization of the GFP expressing islet-cells, at 633 nm with a HeNe—R laser (Model 05LHP925, Melles Griot) for visualization of Exendin-4_(K12)-VT680, and at 748 nm with a red diode laser (Model FV10-LD748, Olympus Corporation, Tokyo, Japan) for visualization of the blood pool agent (AngioSense-780, VisEn Medical, MA). Light was collected using custom-built dichroic mirrors SDM-560, SDM640, and SDM-750, and with emission filters BA 505-550, BA660-730, and BA 770 nm IF (Olympus Corporation, Tokyo, Japan). The Exendin-4_(K12)-VT680 signal was collected sequentially to the other channels to avoid bleed-through into the VT680 channel. A prototypical tissue stabilizer (Olympus Corporation, Tokyo, Japan) was used to reduce motion and to stabilize the focal plane. The stabilizer was attached to the objective and its z-position was finely adjusted using a micrometer screw to apply soft pressure on the tissue.

Histology

Nondiabetic male NOD MIP-GFP mice were cannulated with a tail vein catheter, made using a 30 G×½″ needle inserted into CLAY ADAMS® Polyethylene10 tubing. Subsequently, 200 μL (4 nmol/200 μL) of Exendin-4_(K12)-VT680 was injected via the tail vein catheter. Mice were sacrificed 2 hours after the initial injection, the pancreas was harvested and frozen in OCT compound (Sakura, Finetek, Torrance, Calif.) with isopentane on dry ice. The frozen tissues were sectioned (thickness: 6 μm) and mounted on microscope slides. Without further processing, tissue samples were excited with an EXFO X-Cite™ 120 light source (EXFO Photonic Solutions Inc. Ontario, Canada) and imaged with a Nikon Eclipse 80i microscope with either a 10× Plan Fluor N.A. 0.3 and a 20× Plan Apo 0.75 objective (Nikon corporation, USA) respectively. GFP fluorescence was observed with a Chroma HQ FITC filter cube (HQ480/40×EX, dichroic Q505LP BS, and emission filter HQ535/50m EM) and Exendin-VT680 fluorescence with a Chroma Cy5.5 filter cube (HQ650/45×EX, dichroic Q680LP BS, and emission filter HQ710/50m EM; Chroma Technology Corp., VT, USA). Images were captured with a Cascade Model 512B camera (Roper Scientific, Tucson, Ariz.). Validation of fluorescence signals employed immunohistochemistry for insulin (Insulin (H-86), Santa Cruz Biotechnology, Santa Cruz, Calif.) using the avidin-biotin peroxidase method. The reaction was visualized with a 3-amino-9-ethyl-carbazole substrate (AEC, Sigma Chemical, Saint Louis, Mo.). Adjacent sections treated with nonimmune IgG provided controls for antibody specificity.

Results

In order to evaluate the performance of Exendin-4_(K12)-VT680 in vivo, NOD MIP-GFP mice were anesthetized and the pancreas exteriorized onto a heated chamber for real-time multi-channel confocal imaging. Microvasculature was imaged in the 750 nm channel after injection of AngioSense-780, a long circulating blood pool agent, and islets were identified in the 488 nm channel via their GFP expression. Following this, 8 nmol of Exendin-4_(K12)-VT680 in 200 μL of PBS was injected, and accumulation of the probe in the islets and surrounding tissue was monitored (FIG. 6). Intravital imaging confirmed that prior to imaging no signal corresponding to a pancreatic islet was observable in the Exendin-4_(K12)-VT680 channel (FIG. 7, 0 min). However, after 3 minutes, accumulation of E4_(K12)-Fl could be observed in the pancreatic islets, and this fluorescence increased during the observation time of 20 minutes, after which steady state was reached (Target-to-background ratio of 6:1; FIG. 7). Ninety minutes after injection, the signal displayed still a ratio of 6:1. On comparison of imaging results from the vasculature and GFP-islets, it was confirmed that Exendin-4_(K12)-VT680 co-localized in the pancreatic beta cells, and did not simply extravasate from the vasculature that traverses the islets. During the time of measurement, no increase in background noise from surrounding tissue was observed. The target-to-background ratio of islets over exocrine pancreas was 6.0±0.5 in steady state. To further confirm that probe accumulation occurs in pancreatic beta cells, additional immuno- and fluorescence histology was performed (FIG. 8). These studies also demonstrated exclusive accumulation of Exendin-4_(K12)-VT680 in the islets and specifically in the pancreatic beta cells expressing insulin and GLP-1R. Biodistribution studies showed renal excretion of non-localized agent and intracellular degradation in endosomal compartments.

Example 6. In Vivo Quantification of Beta Cell Mass Using Exendin-4_(X12)-VT750

Animals

Both C57BL/6J (B6) mice and B6.Cg-Tg(Ins1-EGFP)1Hara/J mice (38), which express the green fluorescent protein eGFP within the islets under the mouse insulin 1 promoter (MIP-GFP), were obtained from The Jackson Laboratory (Bar Harbor, Me.). All procedures and animal protocols were approved by the subcommittee on Research Animal Care at Massachusetts General Hospital.

Animal Preparation

Mice were anesthetized with 2% isoflurane and 2 L/minute O₂. The peritoneal cavity was opened with a transverse incision in the disinfected abdominal wall. The gastric-splenic ligament was then dissected and the pancreas carefully exteriorized for intravital microscopy. Robust blood flow was observed in the pancreatic arteries for the duration of each experiment, and pancreas perfusion was confirmed by the presence of a circulating fluorescent vascular agent (Angiosense 680, PerkinElmer, Waltham, Mass.). The exteriorized pancreas was then completely submerged in temperature-controlled lactated Ringer's solution. The temperature near the spleen was carefully monitored using an Omega HH12A thermometer with fine wire thermocouples (Omega Engineering Inc., Stamford, Conn.) and maintained at 37° C. A tail vein catheter, made using a 30 G×½″ needle inserted into CLAY ADAMS® Polyethylene10 tubing, was inserted prior to surgery. Via this tail vein catheter, 200 μL (2 to 4 nmol/200 μL) of Exendin-4_(X12)-VT750 was subsequently injected either with or without pre-injection of Exendin-4 (15 nmol/200 μL).

Intravital Laser-Scanning Microscopy

Images were collected with an intravital laser-scanning microscope (IV100, Olympus Corporation, Tokyo, Japan) using an Olympus 20× (UplanFL N.A. 0.5) objective. Samples were excited at 488 nm with an air-cooled argon laser (Melles Griot, Carlsbad, Calif.) for visualization of either GFP-expressing islet-cells or cells stained with Celltracker Green CMFDA (Invitrogen, Carlsbad, Calif.). The samples were also excited at 748 nm with a red diode laser (Model FV10-LD748, Olympus Corporation, Tokyo, Japan) for visualization of Exendin-4_(X12)-VT750. Light was collected using custom-built dichroic mirrors SDM-560 and SDM-750, and emission filters BA 505-550 and BA 770 nm IF (Olympus Corporation, Tokyo, Japan). The green and red signals were collected sequentially to avoid bleed-through into the VT750 channel. A prototypical tissue stabilizer (Olympus Corporation, Tokyo, Japan) was used to reduce motion artifacts and to stabilize the focal plane. The stabilizer was attached to the objective and its z-position was finely adjusted using a micrometer screw to apply soft pressure on the tissue.

Image Analysis

The accumulation of the Exendin-4_(X12)-VT750 probe within islets was determined by intravital imaging in mouse insulin promoter-green fluorescent protein (MIP-GFP) mice, as described above, using an Olympus UPlanFl 10× (NA 0.3) objective. Images prior to probe injection were collected from islets showing a robust MIP-GFP signal and apparently intact vasculature, immediately following intravenous tail-vein injection of E4_(X12)-VT750 (0.2 nmol/g in 200 μL of a 10 μM solution, PBS). This dose was determined by fluorescence dose response experiments for the fluorochrome chosen. Regions of interest (ROIs) were drawn within and outside the islets and temporal changes were plotted using Prism 5.0a (GraphPad Software La Jolla, Calif.). Ex vivo ROIs were determined using pancreata that were excised 1 hour after IV injection of the Exendin-4_(X12)-VT750 probe into MIP-GFP mice.

Immunohistochemistry and Beta Cell Mass Estimation

The pancreata were embedded in OCT compound (Sakura Finetek, CA) and flash-frozen in an isopentane bath on dry ice. The frozen tissues were sectioned (thickness: 5 μm), mounted on microscope slides and stored at −80° C. For insulin immunohistochemistry, the tissue sections were pre-incubated with 0.3% H₂O₂ to suppress endogenous peroxidase activity and then blocked with 5% normal goat serum in PBS. Following the addition of a rabbit polyclonal insulin antibody (insulin H-86: Santa Cruz Biotechnology, CA), a biotinylated anti-rabbit secondary antibody (Vector Laboratories Inc., CA) was applied. The Vectastain ABC kit (Vector Laboratories Inc., CA) and a 3-amino-9-ethylcarbazole (AEC) substrate (Dako, CA) were then used for color development. All the sections were counterstained with Harris hematoxylin solution. For overall morphology, hematoxylin and eosin (H&E) staining was also performed, and all images were captured using NanoZoomer 2.0RS (Hamamatsu, Japan). Beta cell mass was determined by thresholding and quantitating insulin positive areas in non-adjacent sections at 50 m intervals throughout pancreata and corroborated by insulin stains according to published protocols (Montanya et al. Methods Mol Biol 560:137-158, 2009). All images were digitized using a NanoZoomer 2.0RS (Hamamatsu, Japan). Color deconvolution was based on orthonormal transformation of the original RGB image. For each cohort, we analyzed a minimum of 15 individual whole pancreatic histological slides (3 sections for each sample) containing approximately 40,000×40,000 pixels per image. Image processing was done using Matlab software.

Fluorescence Microscopy of Histological Slides

Fluorescence microscopy was performed at different wavelengths on the same sections of pancreata. GFP fluorescence was observed using a Chroma HQ FITC filter cube (HQ480/40×EX, dichroic Q505LP BS, and emission filter HQ535/50m EM), whereas, Exendin-4 VT750 fluorescence was observed using a Chroma Cy7 filter cube (HQ775/50×EX, dichroic Q810LP BS, and emission filter HQ845/55m EM; Chroma Technology Corp., VT, USA). All images were captured and processed using an epifluorescence microscope, Nikon Eclipse 80i (Nikon Instruments Inc., NY), with a Cascade Model 512B camera (Roper Scientific, Arizona).

Streptozotocin-Treatment

Two cohorts of B6 mice at 14 to 18 weeks of age received intraperitoneal injections of streptozotocin (STZ, Sigma-Aldrich, St. Louis, Mo.) dissolved in 0.1M sodium citrate (pH=4.5); mice received either a single injection of 75 mg/kg or 150 mg/kg body weight respectively or 100 mg/kg STZ daily for 5 days. Mice were given access to 10% glucose-enriched water overnight to avoid hypoglycemia post-injection. Five days following STZ treatment, fluorescence- and histology-based whole pancreas islet quantification was performed. Blood-glucose levels from the tail-vein were measured using the Contour® Blood Glucose Monitoring System (Bayer HealthCare LLC, Mishawaka, Ind.).

Fluorescence-Based Whole Pancreas Islet Quantification

After IV injection of Exendin-4_(X12)-VT750 (200 μL, 10 μM) and a 40-minute time interval, untreated and STZ-treated mice were injected with heparin (approximately 3 units of heparin, 300 μL of a 10 USP/mL solution) 10 minutes before euthanasia by CO₂ asphyxiation. B6 mice used for blocking experiments were injected with Exendin-4 (250 μL, 60 μM, 20-minute time interval) prior to injection with Exendin-4_(X12)-VT750 (200 μL, 10 μM, IV, 40-minute time interval) and then heparin (approximately 3 units of heparin, 300 μL of a 10 USP units/mL solution) 10 minutes before euthanasia by CO₂ asphyxiation. The mice were then immediately perfused with 30 mL of PBS and heparin via the left ventricle. Likewise, 5 mL of PBS-heparin was perfused through the splenic artery in order to completely flush the pancreas. The organ was then excised, weighed, and placed between two cover glass slides using a 1 mm rubber gasket to maintain a constant thickness. Fluorescence reflectance imaging of the entire pancreas was performed using an OV110 epifluorescence imager (Olympus; Center Valley, Pa.) at low magnification (0.14×), equipped with standard filters for the 750 nm channel, and a 2-second exposure. An ROI was drawn around the edges of the pancreas, and the average fluorescence signal was calculated using ImageJ (NIH, USA). For each cohort of mice, the autofluorescence signal in the pancreas was subtracted from both mice receiving injections of only Exendin-4_(X12)-VT750 as well as from the respective mice injected with both Exendin-4_(X12)-VT750 and Exendin-4; specific uptake was then determined by subtracting one from the other.

Laparoscopic Islet Imaging

Mouse pancreata were also imaged with a fiber-optic catheter-based imaging system in live mice using a laparoscopic approach; similar systems are used microendoscopic purposes such as imaging through the pancreatic duct in humans. The in-house designed imaging system includes a 1.6 mm outer diameter fiber-optic catheter, and a 0.9 mm working channel coupled to an optical collection setup containing two high-resolution cameras (Pixelfly; PCO, Germany). Using this system, light collected from the fiber-optic catheter is divided into discrete white light and near infrared (NIR) beams by a 45-degree dichroic mirror. These beams are then focused onto the cameras, which enable simultaneous NIR (excitation/emission 747/780-820 nm) and white light imaging. To perform islet imaging, animals were anesthetized with 2% isofluorane in 1 L/minute O₂, and fixed in a supine position with tape. The pancreas and surrounding organs were then surveyed using the fiber-optic catheter, in a similar manner to laparoscopic clinical procedures. Finally, the scanned area was also imaged using a confocal laser microscope (IV100, Olympus Corporation, Tokyo, Japan), operating at 2 different optical channels (488 nm excitation for MIP-GFP and 748 nm excitation for E4_(X12)-VT750). This was used to confirm that the observed fluorescent areas did indeed colocalize with MIP-GFP expressing cells.

Pharmacokinetics

Under in vivo conditions, Exendin-4_(X12)-VT750 accumulated rapidly and selectively in both islets and beta cells, as confirmed by co-localization with the MIP-GFP signal (FIGS. 9A-D). Pancreata of live MIP-GFP mice were then exteriorized and intact blood flow confirmed through an injection of Angiosense 680. After focusing on an islet, mice were injected with Exendin-4_(X12)-VT750 (2 to 4 nmol/200 μL) and islet signal intensity was measured over time. Exendin-4_(X12)-VT750 was seen to accumulate in pancreatic beta cells within 10 minutes, reaching a plateau at ˜15 minutes (FIG. 9E). At 2 hours after injection the islet signal had increased approximately 64 fold over background. The blood concentration of Exendin-4_(X12)-VT750 followed a biexponential decay, giving a weighted T_(1/2) of 84.4 seconds (FIG. 9F).

The organ distribution of Exendin-4_(X12)-VT750 was estimated by measuring the 750 nm fluorescence in excised organs (mean fluorescence units (MFU)×A²), and comparing them 60 minutes after IV injection to the autofluorescence of non-injected normal mice. The comparative fluorescence distribution was as follows: kidney (69.0%), liver (14.8%), pancreas (7.9%), intestine (3.7%), lungs (2.6%) and stomach (1.7%). The NIR fluorophore uptake in both the spleen and heart was <1%. Serial imaging data suggested that the primary excretion pathway for Exendin-4_(X12)-VT750 was predominantly renal, based on the high quantities of probe in the bladder and kidney; this is consistent with the compound's size and blood half-life.

Validation

To quantitatively correlate probe uptake in pancreatic beta cells, image analysis of MIP-GFP mice was performed. Mice were sacrificed 40 minutes after IV injection of the probe, and their pancreata were removed and sectioned for imaging. Signal in the 488 nm channel (FIG. 10A) represented insulin-promoter-driven GFP expression, an indicator of islet area, while signal in the 750 nm channel (FIG. 10B) read out probe uptake. FIGS. 10C and 10D show the hematoxylin and eosin staining and anti-insulin staining of an adjacent histological slide, respectively. There was an excellent correlation between the islet sizes determined by areas in the GFP and VT750 channels (FIG. 10E, R²=0.992). A similar excellent correlation between Exendin-4_(X12)-VT750 signal intensities and islet areas estimated from GFP expression was found (FIG. 10F, R²=0.944). When mice were injected with unlabeled exendin-4 (15 nmol) prior to injection with Exendin-4_(X12)-VT750 (2 nmol), however, no uptake of imaging agent was observed in pancreatic beta cells. This therefore confirmed the selective binding of the fluorescent reporter to GLP-1 receptors in vivo.

Loss of Beta Cell Mass

To determine whether the probe could also be used to measure loss of beta cell mass, Exendin-4_(X12)-VT750 accumulation (measured via whole-pancreas imaging) and beta cell mass (quantified by histology) were compared, with and without STZ administration to B6 mice. STZ rather specifically kills pancreatic islet beta cells, eventually resulting in insulin-dependent diabetes. FIGS. 11A and 11B show representative examples of islets and surrounding exocrine pancreas tissue at two different resolutions, illustrating that Exendin-4_(X12)-VT750 stains pancreatic islets with high specificity. FIG. 11C summarizes image analysis from various cohorts of mice treated with different amounts of STZ. The NIR fluorescence decreased with increasingly aggressive STZ treatment. Mice were normoglycemic (blood-glucose levels <250 mg/dL) at the lowest STZ dose and hyperglycemic at the higher doses (FIG. 11C). When compared with immunohistochemical estimation of beta cell mass via anti-insulin staining (FIGS. 11D and 11E), there was a good correlation between observed fluorescence intensity for Exendin-4_(X12)-VT750 and beta cell mass (R²=0.854; P<0.0001; FIG. 11E). These data show that probe accumulation correlates with beta cell mass and is sensitive enough to detect a beta cell mass loss in mice that are still normoglycemic (a<25% loss of beta cell mass is detectable at 99% confidence interval; FIG. 11E).

Fiber-Optic Imaging

To test the feasibility of fiber-optic islet imaging and/or confocal microscopy endoscopic imaging, the probe in live mice using a custom-built, single channel (680 nm) micro-endoscopic imaging system was tested (FIGS. 12A-C). MIP-GFP mice were injected with Exendin-4_(X12)-VT750 via tail vein (0.2 nmol/g), and their pancreata were examined. The results showed that the NIR fluorescence signal could be clearly detected, and was co-localized with the GFP signal using a separate confocal microscope but having 2 optical channels (480 and 680 nm; FIGS. 12D-F). This system could thus potentially be used in endoscopic (i.e imaging pancreatic parenchyma through duodenal sweep or via pancreatic duct), laparoscopic or intraoperative settings to map surface located islets.

Example 7. Preparation and Characterization of an Exendin-4-¹⁸F Labelled Conjugate

During earlier studies (Reiner et al. Bioconjug Chem 21: 1362-1368, 2010), the Lys12 amino acid of Exendin-4 (FIG. 13A) was identified to be an ideal attachment point for near infrared fluorophores. Attachment at this site does not perturb the ability of Exendin-4 derived amino acid sequences to strongly and selectively bind to GLP1 receptors.

To facilitate non-invasive whole pancreas imaging of pancreatic beta cells, the synthesis of a ¹⁸F-exendin-4 derived amino acid sequence was envisioned, which could be synthesized based on reported fast and selective labeling strategies (Keliher et al. Chem. Med. Chem. 6: 424-427, 2011; Reiner et al. Angew Chem Int Ed Engl 50: 1922-1925, 2011). Thus, an Exendin-4 based amino acid sequence where the Lys12 was replaced with a cysteine (Exendin-4_(C12)) was synthesized, thus enabling the selective and covalent modification of the thiol-moiety with a maleimide-tetrazine (E4_(Tz12), FIGS. 13B and 13C).

Synthesis of the Exendin-4-¹⁸F Labelled Conjugate

The synthesis of novel maleimide-tetrazine crosslinker 3 was conducted using standard amide coupling conditions (EDCI, Et₃N, RT, (DCM). The synthesis of the tetrazine-modified exendin-4 probe E4_(Tz12) 4 was performed in PBS buffer (pH=7.2) and at room temperature. Finally, the bioorthogonal conjugation of the tetrazine 4 with the ¹⁸F functionalized trans-cyclooctene was performed under aqueous conditions employing DMSO and H₂O at room temperature.

Characterization of the Exendin-4-¹⁸F Labelled Conjugate

FIGS. 14A and 14B show the positive polarized (FIG. 14A) and negative polarized (FIG. 14B) ESI LC/MS spectra of Exendin-4_(Tz12). This modification allowed the conversion of the resulting Exendin-4_(Tz12) with ¹⁸F-TCO or the non-radioactive equivalent, ¹⁹F-TCO. The resulting ¹⁸F-labeled peptide Exendin-4_(TzTCO12)-¹⁸F was purified using TCO-scavenger beads and 3 kDa cut off filters, yielding the pure compound in 82% dcRCY (FIGS. 14A and 14C).

Example 8. In Vivo Assessment of Biological Distribution of Exendin-4-¹⁸F Labelled Conjugate

To assess the biological distribution of the Exendin-4-¹⁸F, 50 μCi of E4_(TzTCO12)-¹⁸F were injected into B6 mice (n=3), the mice sacrificed after 60 minutes, organs excised and the distribution of E4_(TzTCO12)-¹⁸F analyzed using γ-counting (FIG. 15A). The distribution of E4_(TzTCO12)-¹⁸F was comparable to that of the NIR fluorescently labeled peptide E4_(X12)-VT750 (FIG. 15B). Although the total uptake of ¹⁸F was just above 1% of the injected dose, the quantity of the accumulated material was remarkable, given the fact that pancreatic beta cells accounted for just about 0.015% of the total mouse tissue. Taking the mass of the targeted organ into account, the pancreatic beta cells had the highest uptake of E4_(TzTCO12)-¹⁸F of all displayed tissues.

To assess the selectivity of the ¹⁸F-modified peptide, 50 μCi of E4_(TzTCO12)-¹⁸F were injected into MIP-GFP mice (n=3) which express the green fluorescent protein (GFP) under the mouse insulin promoter (MIP), therefore making the pancreatic beta cells detectable via their green fluorescence. Colocalization of the green fluorescent signal in the GFP channel (FIG. 16A) and autoradiography analysis (FIG. 16B) showed remarkable colocalization of fluorescence and activity which did not align with the pancreatic tissue (FIG. 16C). This was understandable since the pancreatic islets are not evenly dispersed in the pancreas.

Confirmation of a) the fact that E4_(TzTCO12)-¹⁸F colocalized with pancreatic beta cells and b) its high specific uptake in beta cells confirmed the results obtained for NIR labeled fluorescent probes, making E4_(TzTCO12)-¹⁸F the first ¹⁸F-labeled GLP1R targeting probe for pancreatic beta cells. 

What is claimed is:
 1. A composition comprising the peptide sequence SEQ ID NO:3, wherein the amino acid at position 12 is cysteine.
 2. The composition of claim 1, wherein the cysteine is modified with a linker comprising a maleimide at a first end and a tetrazine at a second end.
 3. The composition of claim 2, wherein the cysteine reacts with the maleimide via a 1,2-addition reaction.
 4. The composition of claim 2, wherein the tetrazine reacts with a strained alkene attached to a detectable agent via an inverse electron demand Diels-Alder reaction.
 5. The composition of claim 4, wherein the strained alkene is trans-cyclooctene.
 6. The composition of claim 4, wherein the detectable agent comprises a fluorophore, a radioactive isotope, or a paramagnetic element.
 7. The composition of claim 6, wherein the detectable agent is a fluorophore.
 8. The composition of claim 7, wherein the fluorophore is VT-680 or VT-750.
 9. The composition of claim 6, wherein the detectable agent is a radioactive isotope.
 10. The composition of claim 9, wherein the radioactive isotope is suitable for PET imaging.
 11. The composition of claim 9, wherein the radioactive isotope is suitable for SPECT imaging.
 12. The composition of claim 9, wherein the radioactive isotope is selected from the group consisting of fluorine-18, carbon 11, nitrogen-13, zirconium-89, copper-64, gallium-67, technetium-99m, indium-111, iodine-123, xenon-133, and thallium-201.
 13. The composition of claim 9, wherein the radioactive isotope is flourine-18.
 14. The composition of claim 6, wherein the detectable agent is a paramagnetic element. 